加上随身携带的Word8

问题描述

我找不到一个函数 addWithCarry :: Word8 - > Word8 - > （Word8，Bool）已经在 base 中定义。记录为关心进行的唯一功能似乎是 addIntC＃在 GHC.Prim 中，但它似乎永远不会被推送通过不同的抽象层。

通过测试输出值是否在范围内，我显然可以推出我自己的产品，实际上我正在做的是，而是重复使用已经定义好的（可能更有效）。

是否有这样的事情？

解决方案

如果你看，你会发现所有事情都是通过转换为 Word＃取消装箱值并对其执行操作完成的，然后缩小到8位值。我怀疑，比较 Word＃值会更有效率，所以我实现了这样的事情。它的（我发现它比StackOverflow更容易阅读）。

请注意，它包含测试套件和标准基准。在我的系统中，所有各种测试对于盒装版本（user5402的实现）需要约31ns，对于primops版本，约需24ns。

上述lpaste中的重要功能 primops ，即：

  primops :: Word8  - > Word8  - > （W8＃x＃）（W8＃y＃）= 
（W8＃（narrow8Word＃z＃），isTrue＃（gtWord＃z＃255 ##））$ b（Word8，Bool）
 primops $ b其中
z＃= plusWord＃x＃y＃
  

I can't find a function addWithCarry :: Word8 -> Word8 -> (Word8, Bool) already defined in base. The only function documented as caring about carries seems to be addIntC# in GHC.Prim but it seems to never be pushed upwards through the various abstraction layers.

I could obviously roll out my own by testing whether the output value is in range and it's in fact what I am currently doing but I'd rather reuse an (potentially more efficient) already defined one.

Is there such a thing?

解决方案

If you look at the source for Word8's Num instance, you'll see that everything is done by converting to a Word# unboxed value and performing operations on that, and then narrowing down to a 8-bit value. I suspected that doing comparison on that Word# value would be more efficient, so I implemented such a thing. It's available on lpaste (which I find easier to read than StackOverflow).

Note that it includes both a test suite and Criterion benchmark. On my system, all of the various tests take ~31ns for the boxed version (user5402's implementation) and ~24ns for the primops versions.

The important function from the lpaste above is primops, which is:

primops :: Word8 -> Word8 -> (Word8, Bool)
primops (W8# x#) (W8# y#) =
    (W8# (narrow8Word# z#), isTrue# (gtWord# z# 255##))
  where
    z# = plusWord# x# y#

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